In recent years, applications of the GaN-based LEDs have been broadened from such fields as display, indication and key backlight to LCD backlight and illumination, accompanied with an increasingly high luminous efficiency year by year. Since the homoplasmon monocrystal material is rare, the epitaxial growth of the GaN-based material, in general, is made on a heterogeneous substrate. The high-efficient blue and green GaN-based LED growing on sapphire is the most popular technology right now. Moreover, to achieve high-quality epitaxial structure, a u-doped buffer layer is typically inserted between the substrate and the luminous epitaxial layer.
The GaN-based LED chip has two basic structures: lateral and vertical. In the lateral chip, two electrodes are at same side of the chip, easily leading to electrode blockage owning to unequal distances that the lateral current flows in the n-type and p-type cladding layers. In contrast, in the vertical chip, two electrodes are at two sides of the epitaxial layer. The second electrode is the patterned electrode and all p-type cladding layers; therefore, almost all the current vertically flows through the epitaxial layer with rare laterally flowing current, thus improving the current distribution in the plane structure and the luminous efficiency. Moreover, the vertical chip can overcome the shading problem of the p-electrode and increase luminous area of the LED chip.
Referring to FIG. 1 and FIG. 2, the fabrication process of the vertical GaN-based LED chip mainly includes the following steps: growing a u-doped buffer layer and a GaN-based luminous epitaxial layer (sequentially comprising an n-GaN layer, an active layer and a p-GaN layer) on a growth substrate; bonding a conductive supporting substrate on the p-GaN layer, and a p-electrode is stacked on the other face thereof; removing the growth substrate and fabricating an n-electrode. Since the n-electrode contacts the n-GaN layer, it is required to etch the u-doped buffer layer at the bottom to the n-GaN layer through dry etching, which is difficult to control during production. Moreover, the contact resistance is high so that a high component thermal resistance will be derived. When the product is applied in an ultra-high power product, the high thermal resistance may reduce the luminous efficiency and shorten the service life of the component, and further influence the overall performance of the component.